1. Field of the Invention
The present invention relates to an electroplating bath and process for electrodepositing zinc and zinc alloys on a conductive substrate, and more particularly, to an electroplating bath and process incorporating controlled effective additive amounts of a bath soluble compatible quaternary ammonium polymer for enhancing the characteristics of the zinc or zinc alloy electrodeposit.
2. Description of Related Art
Considerable attention has been directed to the development of zinc electroplating baths which will produce zinc deposits of improved quality. For convenience, since zinc and zinc alloy baths are improved by this invention, the term zinc and zinc alloy may be used interchangeably with the application being directed to zinc-iron-cobalt alloys in particular.
Research has been devoted to improving zinc electroplating baths with regard to a number of plating properties such as over-all brightness, absence of pitting, providing of a uniform plating distribution thickness over a wide range of current densities, corrosion resistance, the capability of utilizing high zinc concentrations for increased efficiency, and the providing of a zinc alloy coating which is relatively uniform in composition over the article being plated as a base for further coatings such as a chromate coating.
Alkaline zinc plating baths are generally based on a solution of zinc ions and an excess of a base such as sodium hydroxide and water. High pH alkaline zinc baths however, when used without brightening or addition agents yield deposits which are rough and spongy and are generally unacceptable for most applications.
Zinc electroplating has been conducted in plating baths employing alkali metal cyanide salts which serve in such baths as an additive or complexing agent to achieve the desired plating operation and produce bright, smooth grained zinc deposits. Because of the toxicity of cyanides and environmental considerations however, it is desirable to provide in addition to cyanide baths, plating baths which operate effectively at low cyanide levels or advantageously in the total absence of cyanide salts.
Zinc and zinc alloy electroplating baths of various types have been used in depositing a metal plating of a decorative or functional type on a variety of conductive substrates such as iron and steel to provide for improved corrosion resistance and to enhance the decorative appearance of the article. Typically, zinc and alloys of zinc and nickel, iron and cobalt and mixtures thereof have been used to provide decorative surface finishes while enhancing the resistance of the substrate to corrosion. In many cases, the zinc electroplating coatings are subject to additional corrosion treatments such as a chromate treatment but the composition of the alloy deposited must be uniform over the plated article or the chromate coating will not be satisfactory.
In electroplating processes for example, it is technically difficult to maintain a uniform current density over the whole surface of an article to be plated. Articles having projected portions and recessed portions have a different current density on the projected portion than on the recessed portion and it is difficult to obtain a substantially equal current density on these two portions. The variability of the current density over the article being plated affects the thickness of the plating and uniformity of the alloy components in the coating. This subsequently affects the nature of the chromate coating which may be a bright chromate, a colored chromate, a black chromate, a green chromate, etc. to provide a high corrosion resistant article. A highly desirable bath provides uniform alloy composition deposits and deposits wherein the ratio of the thickness of the deposit as measured at, e.g., 80 amps/ft (ASF) to the thickness of the deposit as measured at 4 ASF approaches 1 with 1 being the ideal thickness ratio since at this ratio the thickness of an article having projected and recessed portions would be the same over the complete article.
It is also important that the zinc plating bath be operable in manual and automatic rack and barrel plating operations and that the chromate conversion coating on the zinc electroplate likewise be operable in manual and automatic rack operations and barrel operations. The chromate coating should be uniform with virtually no iridescence for black chromates.
A number of patents have issued over the years disclosing improved zinc electroplating baths containing specific brightening agents including: U.S. Pat. Nos. 3,853,718; 3,869,358; 3,884,774; 4,113,583; 4,169,771; 4,229,267; 4,730,022; 4,792,038; 5,182,006 and 5,194,140. These baths are generally deficient for a number of reasons.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an alkaline, zinc or zinc alloy electroplating bath which produces bright substantially pit-free zinc and zinc alloy deposits.
Another object of the present invention is to provide an alkaline zinc or zinc alloy electroplating bath which substantially improves the uniformity of the thickness and/or of the uniformity of the alloy components in the plating over a wide range of current densities.
A further object of the present invention is to provide an alkaline zinc or zinc alloy electroplating bath which provides commercially satisfactory zinc and zinc alloy coatings in rack and barrel plating operations.
Another object of the present invention is to provide an alkaline zinc or zinc alloy electroplating bath which provides a zinc or zinc alloy coating which may be chromated producing a full uniform chromate coating coverage with virtually no iridescence when black chromating.
Another object of the invention is to provide an alkaline zinc or zinc alloy electroplating bath which may contain a wide range of zinc concentration levels for different plating operations.
Another object of the present invention is to provide a method to zinc or zinc alloy electroplate substrates using the baths of the invention.
Other objects and advantages will be apparent from the following description.